1. Field of the Invention
This invention relates to fiber-optic communication systems. More specifically, this invention relates to bidirectional signal transmission within such communication systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
In certain remote-guidance systems communication with a guided vehicle is facilitated by an optical fiber linking the vehicle with a control station. The optical fiber is typically wound around a bobbin or secured by other means capable of dispensing the fiber as the vehicle travels downrange. An optical carrier of a first wavelength is used to transport commands from the control station to the vehicle, while digital data from the vehicle is impressed upon an optical carrier of a second wavelength and carried by the fiber to the control station. The rate at which data is transmitted from the vehicle to the control station (downlink data rate) is typically much larger than the converse rate (uplink data rate). In order to facilitate a high downlink data rate the bandwidth of the downlink channel typically spans a much wider frequency spectrum than does that of the uplink channel.
The relatively wide downlink bandwidth in conventional remote-guidance systems necessitates a wide bandwidth receiver within the control station. Unfortunately, the sensitivity of a receiver is inherently reduced upon an increase in signal bandwidth since the noise bandwidth thereof is proportionately enlarged. It follows that the sensitivity of the wideband receiver in the control station constitutes one limitation on the transmission range of conventional remote-guidance systems.
The wavelength of optical energy utilized in signal transmission along the link also affects transmission range. In particular, the loss suffered by light energy propagating within an optical fiber generally decreases as wavelength increases. As conventional fiber-optic communication links within vehicle guidance systems typically include carriers at 850 and 1300 nm. (nanometers) or alternatively at 850 nm. and 1060 nm., optical loss at 850 nm. may be dispositive as to transmission range. In fact, fiber-induced loss at 850 nm. has constrained the transmission range of conventional fiber-optic communication links to approximately 10 Km. Designers of conventional vehicle guidance systems have been precluded from utilizing designs which involve an increase in the wavelength of the shorter wavelength carrier (850 nm.) out of concern for the need to maintain adequate isolation between the uplink and downlink channels.
Hence, a need in the art exists for a fiber-optic communication link wherein transmission range is not limited by the fiber-induced loss at relatively short optical wavelengths.